Tiny air sacs at the end of the bronchioles (tiny branchlike air tubes) in the lungs. They are essential for the process of oxygenating blood and get rid of carbon dioxide (CO2) from the body.
When a person breathes air in, the inhaled oxygen (O2) crosses the thin walls of the alveoli and enters the bloodstream. In an exchange process, Carbon dioxide (CO2), a waste product of the body, passes from the blood into the lungs through the walls of the alveoli, from where it is then exhaled.
In patients with COVID-19, the walls of alveoli become thicker making it more difficult for oxygen to pass through these walls into the bloodstream, and starving the body of required levels of oxygen to sustain life. This can cause Acute Respiratory Distress Syndrome which can be fatal.